Description
The C-series CATs are old enough now that most machines in service have been through at least one cooling system adventure, and the fan motor is usually the part that gets replaced twice. Once for the failure, and once because nobody checked the oil that caused it.
What this part is, in Caterpillar’s own words
179-9778 appears in Caterpillar parts literature as MOTOR GP-PISTON -HYDRAULIC FAN, listed with machine serial prefixes EMR1-UP and MAR1-UP, and shown as a component of the hydraulic cooler and mounting group rather than as a standalone item. The parallel catalogue reference 20R-0121 is listed as a piston motor assembly and appears in Caterpillar’s remanufactured parts programme. Both are fixed displacement piston motors, which matters: a fixed motor has no regulator to fail, so when it cannot drive the fan at speed the cause is internal wear, the pump feeding it, or the oil.
Fitment
Caterpillar 322C, 322C FM, 325C, 325C FM, 325C L and M325C MH material handler, all running the six-cylinder 3126B ATAAC engine. The 325C is a machine of roughly 26,900 kg in standard undercarriage and about 28,600 kg long undercarriage, with net flywheel power of 140 kW (188 hp) to ISO 9249 and 139 kW (186 hp) to SAE J1349, and main implement flow around 235 L/min with a 36 L/min pilot circuit.
Some supplier catalogues extend this number to the 330C. That is wrong: the 330C fan motor is 191-5611 / 20R-0118 and the housing and port arrangement differ. If you have a 330C, use our 191-5611 page instead.
The other half of the circuit
On the 325C the fan motor is fed by a gear pump, catalogued as 200-3406 and listed by Caterpillar as a 10-tooth basic gear pump. It is a separate part with a separate failure mode, and the two are routinely confused because the symptom is the same: the fan does not reach speed. The gear pump loses volume gradually and makes noise as it does so. The motor tends to leak internally and pass flow straight back to tank. Our 200-3406 fan pump page covers the pump side, and the diagnosis sequence is on both pages so you can decide which one you actually need.
Check these before you order a replacement motor
- Spin it by hand with the belts and hoses off. A seized or gritty bearing tells you more than any gauge. Feel for roughness through the whole rotation, not just at one point.
- Measure fan speed against the machine’s specification. If the fan reaches normal speed but the machine still runs hot, the motor is not your problem. Look at the radiator core, the coolant charge and the temperature sensor.
- Check the case drain. Disconnect the drain line, crank or run briefly and catch the flow. A motor throwing large volumes of oil through the drain has internal wear. A drain line that is pinched or routed below the case will destroy a new motor in the same way it destroyed this one.
- Sample the oil from the fan circuit. Dark oil with visible glitter means the previous unit shed material and the whole circuit needs flushing before anything new is fitted.
- Confirm the shaft and flange against photographs. Fan hub adapters and later service fittings vary, and the shaft end is where a wrong part is found out.
Notre hydraulic fan motor diagnosis guide covers the same checks with test equipment, and coolant temperature sensor testing is the five-minute check that stops a lot of unnecessary parts orders.
Cleanliness, because this is where replacements die
Fan motors are the last thing anyone filters. The working target for axial and slipper piston units of this type is around ISO 4406 18/16/13 on Bosch Rexroth’s mobile hydraulics guidance, and several of these CAT fan motors are Rexroth-built units under the Cat catalogue number. Fit a new return filter element with the replacement, and check the suction strainer. If the machine runs hot as well, read why hydraulic oil stays hot after all wear parts are replaced.
Supersession, and how to check it properly
Caterpillar maintains part-number history in its Service Information System, and the practical way to use it is to look the part up against your machine’s serial number prefix rather than against the model badge. Prefix EMR and MAR apply to this item. Parts.cat.com shows status flags including a Replaced marker, and dealers can pull the full supersession chain. If your machine’s book gives a different number, send us the page and we will quote what your machine actually takes. More on how we handle numbers is on our hydraulic fan motor range page.
